Rose black spot is perhaps the most devastating disease of roses in the South. This disease is caused by a fungus (Diplocarpon rosae) that attacks the foliage of many rose varieties in home landscapes.

Many dedicated rose growers battle black spot year after year. The disease can flare up virtually anytime of the year when the leaves remain wet for a period of six or more hours at a time. Frequent rainfall with cloudy days or periods of high humidity can result in disease onset.

The fungus that causes this disease thrives best at 75 to 85 F. The fungus can attach to all types of roses but tends to be worst on hybrid tea roses. Black spot is more of a problem in the landscape than the greenhouse since moisture and temperature can be more easily controlled in the greenhouse environment.

Symptoms:

The most obvious evidence of this disease is the presence of black spots on the individual leaflets. Spots may vary considerably in diameter, but most reach about one-quarter inch. The spots have a “feathery” edge to them (see photos). This is a useful characteristic to identify this disease. Infected leaflets develop a yellow halo around the spots. New spots can appear in as little as five days under wet or humid conditions.

As spots enlarge, the yellow area becomes more obvious . The leaflet yellowing often leads to premature defoliation and subsequent debilitation of the plant. Black spot often results in a decrease in both bloom number and quality. The fungus can also infect the canes. This symptom is not as readily seen as the leaf spots. Cane infections appear as small purple to black blister-like blotches on the canes. These infected canes can harbor the fungus during the winter or during periods of adverse weather conditions. Thus, they are an important source of disease initiation in the spring.

Prescription:

Planting resistant varieties is the most effective method of preventing black spot. The degree of resistance may vary with local environmental conditions. The removal of over-wintering sources of the fungus is a great way to get your control program underway on plants already in the landscape, especially if you have had a problem during the past season. March is the best time to prune out and destroy any dead canes left on the bush over the winter if you missed the opportunity in February. By picking up all of the dead rose leaves under the plant, another source of the disease can be eliminated. Since leaf wetness is important, use of soaker hoses and drip tube irrigation is very helpful.

Fungicide applications may be required in combination with the sanitation practices. Fungicides may need to be applied during most of the growing season on very susceptible varieties that have a history of chronic black spot. Fungicide choices labeled for black spot on roses include those that contain the active ingredients of myclobutanil, triforine, chlorothalonil or propiconazole.

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